This invention relates to tobacco smoke filter elements, more particularly for use as cigarette filters, although the products of the invention may also be useful as filters in other tobacco means, such as cigars, pipes and the like. Since filters for cigarettes are particularly commercially important, the basic embodiments of the invention will be discussed as they relate to filter tip cigarettes.
The prior art is replete with proposals for improved cigarette filters made both from natural products, particularly paper, or synthetic fiber products such as cellulose acetate. An important property of a tobacco smoke filter is its efficiency, that is its ability to remove undesirable constituents from tobacco smoke. Filtration efficiency is generally measured in terms of the percentage of total particulate matter (TPM) removed from the smoke. While filtration efficiency is perhaps the most important property of cigarette filters, other properties must also be considered including pressure drop, taste, hardness, appearance and cost. It is necessary to produce a filter which satisfies commercial requirements in each of these areas.
Frequently, it is necessary to compromise certain properties in order to satisfy the need of others. For example, commonly utilized cellulose acetate filters have a relatively low filtration efficiency since increased efficiency can only be obtained, either by increasing the density of the filter material or the length of the filter element, both of which steps produce a pressure drop across the filter which is excessive and unacceptable from a commercial or consumer standpoint. While various suggestions have been made for the production of filters having improved filtering properties, a considerable number of prior art developments have not been commercially acceptable, either because the resulting filter has been found to have objectionable "taste" characteristics, whereby cigarettes provided with such filters fail to satisfy a large segment of the smoking public, or because the techniques and/or the materials utilized in the production of such filters have increased the cost excessively.
High filtration efficiency has been considered by the industry to be removal of at least 60% of total particulate matter. Cigarette filters having such properties are disclosed, for example, in prior U.S. Pat. Nos. 3,533,416, 3,599,646, 3,637,447, 3,648,711, 3,805,801 3,810,477, 3,811,451 and 3,847,064 all of which are commonly assigned with the instant application. The disclosures of the prior patents are incorporated herein in their entirety by reference.
One particular form of high filtration efficiency cigarette filter which has achieved considerable commercial success is disclosed, for example, in U.S. Pat. No. 4,022,221 which is also commonly assigned herewith and incorporated herein by reference. Broadly stated, cigarette filters in accordance with U.S. Pat. No. 4,022,221 have an inner fibrous cylindrical filter member bonded to a tubular outer member (generally a plug wrap), and the inner member includes both an axially extending elongate central cavity as well as a series of circumferentially spaced outer cavities or channels with the general purpose of improving the flow path of tobacco smoke through the filter and enabling greater utility of the filtering material from which it is made. The patent also discloses methods and means for producing such filters. While filters of the above type are successful in obtaining a high filtration efficiency combined with an acceptable pressure drop, due to the presence of the central cavity in such filters, the harsher smoke tends to be channeled down the center.
It is considered by certain cigarette manufactures that the resistance to flow of smoke should be higher in the center of the filter than at the filter periphery in order to provide improved taste. Standard filters, even those without a central cavity, tend to have a lower flow resistance in the center than at the periphery, because the fibrous material from which the filter is made is frequently compressed, and becomes slightly more dense on the outside of the filter than in the center of the filter.
Other known filters have a higher resistance to flow in the center of the filter, achieved, for example, by inserting plugs or tubes in the central cavity or by varying the density of the fiber mass over the cross-section of the filter with higher fiber densities being provided at the filter axis. Examples of such further filters are disclosed in U.S. Pat. Nos. 4,026,306, 4,046,063 and 4,064,791, all of which are commonly assigned herewith and also incorporated in their entirety by reference. While such filters may provide higher resistance to draw in the center of the filter, some harshness in taste remains.
In the late 1970's, the trend in the tobacco industry was for high filtration filters producing smoke with a low tar content. In the early 1980's, air dilution was used to achieve low tar content without filtration, but this resulted in cigarettes with little taste. There is now a demand for filters having high filtration with little or no air dilution and with the filter preferably having a conventional end appearance. Also, to improve the taste characteristics, there is a demand for a filter having more balanced filtration over the cross-section of the filter. Since conventional cellulose acetate, which is commercially available has a relatively large fiber size, there is no known filter design to achieve the desired filtration levels consistent with the other filter requirements. Certain manufacturers have reverted to paper-type filters which offer smaller fibers and improved filtration, but the problem with this type of filter is that the end appearance may not be acceptable to smokers. Composite filters which overcome this problem are generally not cost effective.